1. Field of the Invention
The present invention relates to RF power splitter apparatus for splitting a radio frequency (RF) electrical signal between two loads, and, more specifically, to an RF power splitter which permits adjusting the ratio of the RF voltage applied to the two loads.
2. Description of the Prior Art
There are many applications in which it is desirable to be able to divide a high power RF signal between two loads and adjust the ratio of voltage applied to the two loads. In some applications, it is important that the voltage split ratio and phase differential between the two outputs of the splitter be substantially independent of changes in impedance of the two loads. For example, when an RF power splitter is used to supply power to two electrodes of a plasma reaction chamber, the impedance of the plasma varies dynamically as a function of changing operating conditions such as gas pressure in the chamber, temperature, and RF power level. To maximize the stability of the plasma, the two output voltages of the RF power splitter should have as little sensitivity as possible to changes in the two load impedances it is driving.
Examples of plasma reactors having RF power splitters for supplying power to two electrodes are U.S. Pat. No. 4,871,421 granted Oct. 3, 1989 to Ogle et al. for a "Split Phase Driver for Plasma Etch System", and U.S. Pat. No. 4,887,005 granted Dec. 12, 1989 to Rough et al. for a "Multiple Electrode Plasma Reactor Power Distribution System".
Known RF power splitters include RF transformers in which wipers or multiple taps permit adjustment of the voltage split ratio. However, this requires the use of mechanical switch contacts or wipers which suffer deterioration over time.
Other known RF power splitters consist of networks of inductors and capacitors in which one or more of the inductors or capacitors is variable so as to adjust the ratio of voltage applied to two loads. However, in known networks of this type, the voltage split ratio and the phase differential between the two output signals generally are very sensitive to the impedances of the loads. Also, in such networks the relative phase angle between the two output signals changes substantially as the power split ratio is varied.
There is a need for an RF power splitter which can adjust the ratio of voltage applied to two loads over a wide range without imposing excessive changes in the relative phases of the two output signals and with minimal sensitivity to changes in the impedances of the two loads.